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 Forum index » DIY Hardware and Software » The Repair Shop
Non electro caps failing?
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bunker



Joined: Mar 19, 2009
Posts: 57
Location: Manchester, UK

PostPosted: Thu May 16, 2013 2:24 am    Post subject: Non electro caps failing?
Subject description: Do non electrolytic caps fail over time?
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Hi all. A question that's been puzzling me and is related to a synth I own. Do film/polyester/polystyrene caps tend to fail over time? I know electro's can 'dry out'/short out etc. But this is due to the electrolyte inside them degrading isn't it?

The back story is this. I picked up a Crumar Bit 99 synth years ago as a non runner. I can't remember exactly what was wrong with it but to cut a long story short I got it 99% working. However there is a very obvious fault which sadly renders the instrument unusable as a six voice synth as one of the voices is displaying a rather odd problem. The voice doesn't articulate correctly and I assumed it was a duff electro cap in the EG circuit for that voice. Attack is fine but after holding the note you get a very sudden closing of the amp/filter as if there is a fault with the EG. Imagine holding a note while powering down..That's the envelope response I get for that one voice Rolling Eyes

I've not got the synth/schematics in front of me right now but from memory the EG's are software based and the voices are CEM3379 chips. I have swapped the chips around but the fault still remains.

I'm assuming that even though the envelopes are software based there will be at least one cap that charges/discharges at a rate determined by the EG? Or could I have a corruption in the software of the EG for that one voice? That I find hard to believe? Either way I'm pretty sure I replaced all the electro caps that looked to possibly be part of the circuit. There are other non polarised film/poly caps in the circuit and I was wondering, before I replace all those, if they can degrade/fall over like electro's can? As I say I don't have the schematic to hand and I'm pretty sure the other caps will be filter caps etc. and not directly related to the EG's.

I'm gonna possibly have another look tonight if I get time; the last time I had a play around wit it must have been 18 months ago so forgive me if I sound a little vague!

If any of you guys have had envelope issues with polysynths shout up! We might find some common ground!! I can't wait to get this fault nailed down as the synth sounds wonderful until you get the dreaded squelch/thud of the voice in question. I suppose it adds a bit 'movement' to a chord but its not something I want all the time!!

Thanks in advance.
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elmegil



Joined: Mar 20, 2012
Posts: 1344
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PostPosted: Thu May 16, 2013 5:17 am    Post subject: Reply with quote  Mark this post and the followings unread

FWIW this is not a definitive data point, just an anecdote....

I picked up a non-working S-10. Initially it made nothing but a high pitched whine.

I replaced a handful of mylar caps, after which point it was working except for the data disk.

So I would say yes, non electro caps will also go bad over time.
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bunker



Joined: Mar 19, 2009
Posts: 57
Location: Manchester, UK

PostPosted: Thu May 16, 2013 5:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Cheers elmegil, a complete re cap in the suspect area sounds like a plan. It's a keeper so its worth it to me to do it. Thanks for your response, I'll keep you informed of how I get on. It's so infuriating having 5 good voices and one duffer!! From what I've heard up to now the Bit can sound really nice. Designed my Mario Maggi of Elka Synthex I believe? Either way prices are rising so a replacement unit is looking less and less likely so whatever the fault is I'll get to the bottom of it!!
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mph



Joined: Aug 25, 2007
Posts: 66
Location: France

PostPosted: Tue Jun 04, 2013 12:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:

So I would say yes, non electro caps will also go bad over time.


That's true! I had many tropical fish caps completely dry and broken, same thing with some old mylar caps, plus their "vintage" tolerances are really wide compared to the modern production.
So it's always a good thing to do a whole recap on a faulty board.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue Jun 04, 2013 1:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

What in the world is a tropical fish cap? Smile
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mph



Joined: Aug 25, 2007
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Location: France

PostPosted: Tue Jun 04, 2013 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

This stuff Wink :
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Some people claim they have a special MOJO (hopefully mostly in old fuzz pedals)... err I say they are crap.
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elmegil



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PostPosted: Tue Jun 04, 2013 2:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

Interesting. I've never seen those. Thanks for the info Smile
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yusynth



Joined: Nov 24, 2005
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PostPosted: Wed Jun 19, 2013 4:26 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Interesting. I've never seen those. Thanks for the info Smile


Hopefully these are no longer available Wink

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Yves
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elmegil



Joined: Mar 20, 2012
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PostPosted: Wed Jun 19, 2013 7:00 am    Post subject: Reply with quote  Mark this post and the followings unread

yusynth wrote:
elmegil wrote:
Interesting. I've never seen those. Thanks for the info Smile


Hopefully these are no longer available Wink


True enough Smile

I sometimes work on broken commercial gear though, so it is good that I won't be confused by them if I see them Very Happy
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Dragon's Lair



Joined: Dec 29, 2006
Posts: 185
Location: Nakusp BC, Canada
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PostPosted: Wed Jun 19, 2013 8:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Old tantalum's are prone to failure also.

Dave
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bunker



Joined: Mar 19, 2009
Posts: 57
Location: Manchester, UK

PostPosted: Thu Jun 20, 2013 9:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Cheers guys! Work commitments have kept me away from the bench lately.
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filterstein



Joined: Mar 17, 2007
Posts: 42
Location: The Netherlands

PostPosted: Sat Jun 22, 2013 6:01 am    Post subject: Reply with quote  Mark this post and the followings unread

If the eg is software based i'd expect a cap to hold the voltage for the vca cv steady. This cap is usually somewhere between a multiplexer chip behind the Dac and a buffer opamp.
The cap should be large enough to keep the cv steady until the next software update cycle outputs a new value.
If that cap is bad it could cause what you experience.
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analog_backlash



Joined: Sep 04, 2012
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PostPosted: Fri Jun 28, 2013 3:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

mph wrote:
This stuff Wink :
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Some people claim they have a special MOJO (hopefully mostly in old fuzz pedals)... err I say they are crap.


I'm really late reading this, but I'm glad someone else has said what I thought! They're Mullard C280s and they seem to sell for ridiculous prices on ebay. I have quite a few of them myself and I've been tempted to sell them to gullible pedal makers...

Gary
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bunker



Joined: Mar 19, 2009
Posts: 57
Location: Manchester, UK

PostPosted: Sat Jun 29, 2013 12:29 am    Post subject: Reply with quote  Mark this post and the followings unread

filterstein wrote:
If the eg is software based i'd expect a cap to hold the voltage for the vca cv steady. This cap is usually somewhere between a multiplexer chip behind the Dac and a buffer opamp.
The cap should be large enough to keep the cv steady until the next software update cycle outputs a new value.
If that cap is bad it could cause what you experience.


Exactly what I thought. I'm presuming there would be 6 caps (one for each vca eg) and from past experience these eg caps are usually electro's or polystyrene hence the original question. As soon as work commitments allow ill get the board back on the bench and continue my re capping mission!
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